The various optical fibers contained in a single submarine cable need not necessarily all have the same starting point or the same destination. Underwater "branching units" are therefore provided for interconnecting a plurality of cables in order to constitute a submarine network providing the desired point-to-point links.
Such an underwater branching unit may additionally contain:
opto-electronic regenerators for amplifying the optical signals transiting along the fibers;
remote surveillance devices for monitoring the regenerators and serving to locate possible faults; and
remotely controlled switching devices acting either optically between fibers or else electrically between regenerators, for example to divert signals if a normal path is down.
All of this assumes that an electrical power supply accompanies the optical cables.
Remotely controlled electrical switching devices are added thereto in order to modify the configuration of the remote power supply circuit in the event of an untimely local break therein.
These arrangements are going to be used for the so-called TAT8 transatlantic link which is to be laid in 1988.
Although known solutions are of sufficient reliability, they do not give entire satisfaction under all configurations. In order to optimize the utilization of each optical fiber, it is desirable to perform "underwater multiplexing" making it possible to distribute traffic conveyed by a single incident fiber to a plurality of destinations, and vice versa.
Such multiplexing may be synchronous. In which case the transmitted signal is at a high data rate and is locked to a single clock rate defined by a master clock. The signal is given an appropriate frame structure and may optionally have a line code which is suitable for transmission through regenerators, or which is intended to facilitate the remote surveillance of the regenerators.
The framing and the line code may be applied to the signal at land-based transmission stations. In outline, such stations receive N quasi-synchronous digital input streams, synchronize the streams by adding a suitable number of bits, and then combine them with the frame structure (and optionally the line code) suitable for obtaining the high data rate signal to be transmitted. The signal is constituted by a sequence of "segments" each corresponding to one of the input streams, and it includes synchronizing elements enabling the segments to be identified.
The underwater multiplexing operations may be performed electrically in modified branching units.
In a "parallel" scheme, each incident fiber is connected to a N-outlet demultiplexer; each outlet fiber is connected downstream from a N-inlet multiplexer; a fixed or reconfigurable matrix defines the transfer relationship between the N demultiplexer outlets and the N multiplexer inlets.
In another scheme, which is referred to herein as a "serial" scheme the basic component is a drop and insert switch. Such a switch extracts all of the segments corresponding to a given stream from the high data rate signal, and replaces them by segments coming from another stream. One or more intermediate storage memories need to be provided. Finally, in addition to the appropriate number of drop and insert switches for establishing all of the desired interchanges between the base streams (up to N per pair of fibers), the complete equipment requires a distribution matrix for defining these interchanges exactly.
Whichever one of these two schemes is used, the resulting branching unit is complex. In addition to the complexity due to the underwater multiplexing equipment per se, it is also necessary to provide regenerators, remote surveillance devices associated therewith, and remote power supply electrical switches (which are difficult to install anywhere else). Further remote control and remote surveillance devices need adding for the underwater multiplexing equipment itself. All of this must be provided with the redundancy required by the underwater location. Finally, device must be provided which is capable of by-passing the underwater multiplexing equipment in the event of a fault, in order to maintain traffic.
All of this presents a considerable problem both for technical and for economic reasons.
Technically, the assembly is difficult to design and build in the form of a housing that can be handled at sea; account must be taken of the danger of interference between the subassemblies and this danger is made worse by the high data rate and by problems of heating and energy dissipation. It may also be observed that there are considerable differences in potential between subassemblies, since a large number of circuits are powered using a limited remotely fed current, and the architecture of the remote power supply system requires considerable dielectric insulation which is not very compatible with obtaining low thermal impedance.
Economically, such apparatus would need to be designed on a case-by-case basis for each application. Economies of scale could only be expected for the basic components, but never for the complete apparatus, and this drawback would apply from the drawing board all the way to the stocks required for maintenance purposes.
The present invention provides a solution to this problem.